Anchor DLC purchases to a base game instead of a parent purchase

Add-on purchases (DLC, Season Pass, Battle Pass) previously linked to a
parent *purchase* via the `related_purchase` self-FK. When the base game
was bought inside a multi-game purchase (e.g. a bundle), there was no
per-game purchase to point at — only the whole bundle.

Replace it with a `related_game` FK (Game -> Game): an add-on belongs to
a *game*, which is unambiguous regardless of how the base game was bought.

- models: drop `related_purchase`; add `related_game`
  (SET_NULL, related_name="addon_purchases"); require it for non-GAME
  types in `save()`.
- forms: replace the parent-purchase picker with a flat `related_game`
  game search (reusing SearchSelectWidget/_game_options); drop the now
  unused related_purchase_queryset/RelatedPurchaseChoiceField.
- views/urls: remove the obsolete related_purchase_by_game endpoint.
- add_purchase.js: drop the parent-dropdown refetch; keep platform
  auto-fill; retarget the type toggle to #id_related_game.
- migration 0020: add -> backfill (related_game = parent's first game by
  sort_name) -> remove related_purchase.
- tests: model validation unit tests + an e2e test for the flat picker.

related_game is deliberately game->game so it can later be synced from
IGDB's parent_game without schema changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

games/views/purchase.py

@@ -393,25 +393,3 @@ def finish_purchase(request: HttpRequest, purchase_id: int) -> HttpResponse:
         game.status = Game.Status.FINISHED
         game.save()
     return redirect("games:list_purchases")
-
-
-def related_purchase_by_game(request: HttpRequest) -> HttpResponse:
-    games: list[str] = request.GET.getlist("games")
-    if games:
-        from games.forms import related_purchase_queryset
-
-        form = PurchaseForm()
-        qs = (
-            related_purchase_queryset()
-            .filter(games__in=games)
-            .order_by("games__sort_name")
-        )
-
-        form.fields["related_purchase"].queryset = qs
-        first_option = qs.first()
-        if first_option:
-            form.fields["related_purchase"].initial = first_option.id
-        return HttpResponse(str(form["related_purchase"]))
-    else:
-        # abort swap
-        return HttpResponse(status=204)